1. Field of the Invention
This invention relates to an improved method of preparing hydrophobic oil droplet-containing capsules composed of a urea/formaldehyde resin wall membrane. More particularly, it relates to a method for preparing microcapsules comprising polymerizing urea and formaldehyde in the presence of an anionic polyelectrolyte and forming a wall membrane of urea/formaldehyde resin that encircles hydrophobic oily liquid droplets, wherein the method is conducted in the presence of an ammonium salt of an acid for the purpose of improving the characteristics of the wall membrane and preventing coloration of the capsule liquid slurry when the microcapsules are to be used for, e.g., a pressure sensitive copying paper and the microcapsules contain a color former.
2. Description of the Prior Art
A method for preparing microcapsules by polymerizing urea and formaldehyde in the presence of an anionic polyelectrolyte and forming a wall membrane of urea/formaldehyde resin encircling dispersed hydrophobic oily liquid droplets is disclosed in Japanese Patent Publication No. 12518/63 and Japanese Patent Application (OPI) No. 9079/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
However, the wall membrane of the microcapsules prepared by this method has low thermal and strength properties whereas the permeability of the wall membrane is high. What is more, if this microcapsule is filled with an oily solution of a color forming agent for use in a pressure sensitive recording paper, the capsule solution slurry is subject to coloration, which tendency increases in intensity as the pH of the system is decreased.
Accordingly, if oil-containing microcapsules prepared using such a conventional method are employed in a pressure sensitive recording paper, the wall membranes of the microcapsules are thermally degraded with the color forming ability of the paper being reduced when it is stored at high temperatures.
Due to the low strength of the wall membranes, the microcapsules are easily ruptured with the slightest external pressure, and thus the ease of handling during processing or printing is lowered.
In addition to the above, the high permeability of the wall membranes often causes an excessive coloration of a developer sheet or microcapsule sheet when the sheets come into contact with water.